1. Field of the Invention
This invention relates generally to apparatus for the testing of integrated circuits (ICs) and more specifically to a probe testing system comprising a formed membrane including a multi-layer flex circuit for contacting chips with large numbers of contact pads.
2. Description of the Prior Art
Semiconductor integrated circuit (IC) devices are typically manufactured from thin, round silicon wafer slices as large as ten inches in diameter. Each such wafer can yield hundreds of IC devices. After a combination of masking, diffusion, and etching steps, the individual wafers are diced into individual chips, or dice, for further processing and packaging. Unnecessary expense can be avoided by identifying defective chips as early in the fabrication process as possible, especially before the final step of packaging. Defective chips are typically identified by testing each of the circuits on a wafer with a probe that is able to make contact with the rows of connection pads on the perimeters of the respective chips. Chips that do not pass a series of tests are marked and later discarded. Therefore, only operational chips will reach the additional manufacturing steps and concomitant expenses of wire bonding and plastic molding in device packages.
Recent breakthroughs in packaging technology have overcome traditional barriers that existed which severely limited package pin counts. It is now possible to economically construct an IC with a hundred or more input/output pins. Each such pin has a corresponding pad on a bare chip that is probed during interim testing.
The testing operation may be performed at a wafer level before the wafers are sawn apart into individual chips. A test system typically comprises a test controller for executing and controlling a series of test programs, a wafer dispensing system for mechanically handling and positioning wafers in preparation for testing and a probe card for maintaining an accurate mechanical contact with the device-under-test (DUT). The probe card further provides an electrical interface between the test controller and the DUT and includes a printed circuit board (PCB), which is sometimes referred to as a performance board. Performance boards are typically customized for individual ICs or IC families. The probe card comprises a plurality of test probes positioned to accurately coincide with the input/output (I/O) pads of the DUT.
The test controller generates a series of patterns of test signals that include specific combinations of voltages and currents which stimulate a DUT. The test signals are coupled via the performance board and its test probes. A particular chip's responses to the test signals are input by the probes and coupled back to the test controller via the performance board. The voltage, current and/or frequency responses from the DUT are monitored and analyzed. The response signal patterns are typically compared with a set of predetermined patterns. Chips which are determined not to have met predetermined testing criteria are culled from the lot, the remainder of the chips pass on for further processing.
One type of conventional wafer probe comprises many fine styluses, or probes, mounted on a performance board. The tips of such probes are positioned to contact the I/O pads of a single DUT chip. The opposite ends of the probes are typically soldered to traces of printed circuits on the performance board and couple DUT signals to the test controller. A wafer dispensing system delivers a wafer to be tested to a position under the probe card, aligns the wafer and raises it until electrical connection is established between the probes and the I/O pads of a DUT.
A membrane probe technology for wafer probing typically includes an array of micro-contacts ("contact bumps") on a protruding part of a thin, flexible dielectric film membrane. The extent of the protrusion is referred to as the "probe depth". A microstrip transmission line is formed on the membrane for each contact bump electric connection to the performance board. Individual contact bumps are typically formed by a metal plating method. Photolithographic methods are conventionally used to produce the microstrips.
U.S. Pat. No. 5,180,977, entitled, "MEMBRANE PROBE CONTACT BUMP COMPLIANCY SYSTEM," by Richard E. Huff, describes one such membrane probe technology. Two other U.S. Pat. Nos. 4,906,920, and 4,918,383, both to Richard E. Huff, et al., also describe membrane probes. The '920-patent describes a membrane probe which has a self-leveling mechanism to improve the surface coplanar alignment between a membrane probe and a DUT. An elastomeric bed is used to cushion the force of the probe on the DUT. The '383 patent describes a membrane probe with an automatic contact scrub action obtained from a fixed leaf spring and two variable leaf springs that create an asymmetry which causes a probe card to move laterally as a set of contact bumps engage a wafer.
Membrane probes thus need no extending needles or blades to hold the fine probe tips in place, because the contact bumps can be formed directly on a supporting membrane, unlike the conventional probes. The contact bumps permit large numbers of contacts with high probe density. Improvements in mechanical and electric performance may also be realized by the membrane probes because of the simplicity of its configurations. Typical support membranes are comprised of conventional flex circuit with etched copper traces.
A prerequisite for successful IC testing by either membrane or conventional probe cards is the establishment of proper electrical contact between the probes and I/O pads on a DUT chip. In practice, a probe card and its probe tips or contact bumps may not be exactly coplanar with the surface of the DUT's I/O pads. In the case of membrane probe cards, a self-leveling system is used to accommodate this non-coplanarity condition. To compensate for the same such variations with conventional probe cards, a wafer dispensing system is controlled to raise the wafer a predetermined distance beyond the first point of contact to force a proper contact with all the probes. Such practice is generally referred to as "overdrive".
Nonconductive oxide films typically form on the surface of the I/O pads on a DUT and can interfere with good testing. Such film layers are typically only five to ten nanometers thick, but a concomitant high degree of resistance nevertheless substantially impairs current, voltage, and/or frequency response measurements. Conventional probe cards resolve such problems by tilting each probe at a slight angle with respect to the plane of the DUT. Thus, as the probes are deflected by contact with a wafer, each probe skids along the surface of the corresponding I/O pad. This small horizontal movement produces a scrubbing action that is sufficiently vigorous to remove the surface oxide film on a typical DUT. Membrane probe incorporate a scrubbing motion by using a set of flexure pivot assemblies.
The probes and the membrane with its contact bumps are therefore subjected to the forces of vertical overdrive and horizontal pulling and scrubbing in the test operations. These stressful operation conditions can cause the membrane to lose resiliency. Material deterioration's and structure break down can also occur which cause premature damages of the probe card.
The contact bump area of a typical membrane probe must protrude out to a probe depth of approximately ninety mils from the general plane of a probe card, such that only the contact bumps contact a wafer DUT. With limited numbers of contacts to be made and a resulting use of non-multi-layer circuit membranes, it has been practical to form the membrane protrusion by stretching the membrane under controlled heat and pressure conditions.
High DUT pin counts and corresponding high number counts of contact bumps have dictated the use of multi-layer flex circuit materials for use as probe membranes. Multiple circuit layers are needed to carry the large number of signal wires necessary and advantage can be taken of power-planing and ground-planing techniques. But such multi-layer membranes cannot be stretched to the necessary probe depths because the membranes are too rigid.